The use of metal-oxide-semiconductor (MOS) devices has long been observed in microprocessors, microcontrollers, static RAM, and other digital logic circuits. This can be unsurprising given that MOS devices typically boast high noise immunity and low static power consumption. The physical ‘metal-oxide-semiconductor’ structure of MOS devices can be a reference to the structure of certain field-effect transistors (FET) where a metal gate electrode is disposed on top of an oxide insulator and where the oxide insulator can reside on top of a semiconductor material.
FETs are typically unipolar transistors that utilize an electric field to control electrical behavior and can involve single-carrier-type operation. The conductivity of a FET between the drain and source terminals can be controlled by an electric field. The electric field can be generated by the voltage difference between the source and gate of the device.
Metal-oxide-semiconductor field-effect transistors (MOSFET) are typically a type of field-effect transistor and typically comprise an insulated gate where the voltage applied to the gate can determine the conductivity of the device. The ability to vary the conductivity of a MOSFET device can render MOSFETs particularly suited to, for example, amplifying or switching electronic signals.
It will be appreciated that extrinsic and/or ‘doped’ semiconductor devices can be prevalent in the art. A ‘doped’ semiconductor device can refer to a semiconductor where a doping agent has been introduced to differ the electrical properties of the semiconductor to mimic electrical properties of an intrinsic or ‘pure’ state. Doping can be achieved through the addition of atoms, also typically referred to as impurities, which can change electron and hole carrier concentrations of the semiconductor at thermal equilibrium. The dominant carrier concentration in an extrinsic semiconductor can define whether the semiconductor is an n-type or p-type semiconductor, where an n-type semiconductor typically has a larger electron concentration and a p-type semiconductor typically has a larger hole concentration.
In some FETs, for example an n-channel enhancement mode transistor, a conductive channel typically is not created during manufacturing and a gate-to-source voltage is typically necessary to create the conductive channel. The application of a positive voltage between the gate and the source can act to attract free-floating electrons within the FET towards the gate forming a conductive channel Typically, sufficient electrons are first attracted near the gate to counter dopant ions in the FET and this can occur with the formation of a depletion region. A depletion region can be a region where mobile charge carriers have been forced away by an electric field leaving substantially only ionized donor or acceptor impurities.
The voltage at which a depletion region forms in an FET can be referred to as the threshold voltage of that FET. This can be the minimum gate-to-source voltage differential that can be required to, for example, create a conducting path between source and drain terminals. Notably some other FETs, such as n-channel or p-channel depletion mode FETs, can have pre-existing conductive channels. In such other FETs, the voltage threshold can denote the voltage level at which the conductive channel is wide enough to allow majority carriers to flow easily.
A voltage multiplier can be an electric circuit which typically converts AC electrical power from a lower base voltage to a higher DC voltage using, for example, a network of capacitors and/or diodes. In voltage multipliers, the periodic direction cycling of AC electric power can be aligned with specifically orientated diodes to, for example, selectively charge a series of capacitors in succession. The number of capacitors used in a voltage multiplier can dictate a peak voltage obtainable with the multiplier.
A DC-to-DC converter can be an electric circuit capable of converting a source of DC electric power from one voltage level to another. One type of DC-to-DC converter is a charge pump which can use capacitors as energy-storage elements to achieve a higher or lower voltage power source. Unlike typical AC voltage multipliers, charge pumps can feature a switching device to, for example, control connection to the capacitors. By way of example, a simple two-stage charge pump may in its first stage feature a capacitor connected across a supply until it is charged to the voltage of said supply. In the second stage the switching device can reconfigure the circuit such that the capacitor is placed in series with the supply to the load. In the absence of non-ideal effects, the function of this circuit can be to double the voltage supplied to the load.
The Dickson charge pump, or Dickson multiplier, is a type of DC-to-DC converter arrangement. As is understood by one of ordinary skill in the art, the ideal output voltage from a Dickson multiplier is typically unachievable. The threshold voltage of the DC-DC converter is typically known to contribute to a reduction in ideal voltage output, and there can be parasitic capacitances at or between each component in the converter. An increase in switching clock frequency can ameliorate output voltage drops and can further reduce the size of the capacitors that can be needed due to, for example, a reduction in charge stored per cycle. An increased clock frequency can eventually lead to losses due to, for example, stray capacitance and/or other imperfections. Clock frequency increases can be beneficial to a point.
Dickson multipliers are typically used in integrated circuits where it is desired to increase a voltage battery supply that is lower than the voltage needed by the circuit to function. Such integrated circuits are typically designed to feature MOSFET(s) that are wired to behave like diodes rather than diodes themselves. As diode wired MOSFETs do not typically work well at very low voltages, more complex circuits are often used to overcome this deficiency. One such example features a second MOSFET connected in parallel with the switching MOSFET biased to its linear region. In this example, this second MOSFET has a lower drain-source voltage than the switching MOSFET has on its own and acts to increasing output voltage.
SONOS, or Silicon-Oxide-Nitride-Oxide-Silicon, can be commonly used as non-volatile computer memory and can be a charge trap flash variant. SONOS can be distinguished from other types of flash and MOSFETs because of its typical use of silicon nitride film as opposed to polycrystalline silicon that can be used. The use of silicon nitride film for charge storage can confer numerous advantages over polycrystalline silicon, including improved reliability and/or higher yield due to a reduced susceptibility to defects in the tunnel oxide layer.
It is therefore an object of embodiments of the invention to incorporate SONOS devices into voltage multiplier or charge pump circuits. It is a further objective of embodiments of the invention to directionally alter the threshold voltage of SONOS devices in a single direction, as explained herein, to achieve additional efficiency improvements.